It is often desirable to prevent (or at least reduce the probability of) detection or reception of radio communications signals. A number of techniques for low probability of detection communication have been used and proposed over the years, although spread spectrum techniques have generally achieved the most success in this regard. In a spread spectrum system, a spreading code is used to greatly expand the bandwidth of transmitted signals. The transmitted signals thus have a relatively large bandwidth as compared to the bandwidth of information encoded into the transmitted signals. For intended receivers, the spreading code can be known, and thus can be removed once synchronization has been obtained. The process of removing the spreading code in the receiver can also provide benefits in reducing the effects of interference and/or jamming. For adversaries who lack knowledge of the spreading code, detection of and synchronization to the transmitted signal can be difficult. Jamming a spread spectrum signal can also be difficult, since without knowledge of the spreading code, the intended receiver gains an advantage over the would-be jammer.
Two different types of spread spectrum systems are frequently used: frequency hopping and direct sequence. In frequency hopping, the transmitted frequency is discontinuously changed (frequency hopped) at a regular interval. The spreading code is used to determine the frequency during each hopping (dwell) interval. In direct sequence spreading, the transmitted signal is directly modulated by a spreading code (chip sequence) which changes at a much faster rate than the data encoded into the signal. Various types of modulation can be used, including for example, phase shift keying and M-ary frequency shift keying.
Spread spectrum systems, however, have various vulnerabilities to different types of detectors. In particular, it can sometimes be possible for an adversary to reliably detect the presence of a spread spectrum transmission without knowing the code. When it is necessary or desirable for the presence of spread spectrum transmissions to be covert, such vulnerabilities are undesirable.